The present invention relates to a process for crystallizing an adduct of 2,2-di(4-hydroxyphenyl) propane and phenol in aqueous phenol, under vacuum.
It is known that in order to obtain 2,2-di(4-hydroxyphenyl) propane (hereinafter referred to as diphenylolpropane or DPP) having a suitable purity for certain applications such as the preparation of polycarbonate resins, it is first necessary to purify the raw product. One such prior art method consists of crystallizing diphenylolpropane in a suitable solvent such as aqueous phenol. Thus, in U.S. Pat. No. 3,192,270, a process for the manufacture of pure di(hydroxyphenyl) alkanes is described which comprises dissolving these compounds in hot aqueous phenol and cooling the solution thereby precipitating crystals of the adduct of di(4-hydroxyphenyl) alkane and phenol and liberating the phenol from the adduct.
From a practical standpoint, the aforementioned process comprises the following steps: melting of the mixture resulting from the condensation of phenol and a ketone under the action of an acid catalyst, which contains the impure phenol-diphenylolalkane adduct as well as phenol; neutralizing the mixture with a concentrated aqueous solution of an alkali; separating the resulting aqueous phase; adding sufficient water to the remaining organic phase to saturate all of the phenol contained therein; decanting the mixture into two phases; separating the aqueous phase; cooling the organic phase to room temperature to allow the purified adduct of diphenylolalkane-phenol to crystallize and, lastly, separating this adduct from the mother-liquor. This process typifies prior art processes for the crystallization and purification of diphenylolpropane and it is apparent that this prior art process is overly complicated and requires excessive amounts of water.
Therefore, there exists a need for a more efficient and economical process for crystallizing diphenylolpropane.